A variety of combination drill screws are known in the art. Such drill screws typically include a head shaped to permit torquing of a shank having a threaded section with a drill bit formed at its leading end. The drill bit is used to produce a hole into a work piece upon rotation of the shank in a clockwise direction.
Drill screws are used to fasten components where drilling is necessary to first produce a hole yet speed necessitates quick attachment or ease of assembly seeks to eliminate the drilling step during attachment. The drill bit formed on the end of the shank is of a length suitable for boring a hole through a wall or other attachment fixture. Thus, drill screws simply allow fastening in a single operation rather than a two or three step process; that is, separately drilling a bore hole and torquing in a screw. For a number of applications, the conventional drill screw has proven inadequate. For instance, attempts at securing a railing bracket or other such object to a fiberglass boat hull using prior art drill screws often results in cracks propagating from the bore hole as the threaded section of the shank is screwed into the hole. Such cracks can be cosmetic in nature and mar the aesthetic qualities of the work piece, such as cracks formed in the gel-coat layer used to coat the fiberglass hulls of boats. However, cracks may have also affect the structural integrity of the screw anchorage by allowing the threaded section to loosen and back or fall out of the bore hole. To prevent such cracks from forming, a pilot hole must be drilled completely through the work piece forming a clean hole for insertion of the screw, thus making the conventional screw drill inadequate. It is imperative that the drilled hole is completely formed before insertion of the screw. If even a small portion of hole is not completed, insertion of a screw will cause pressure to be formed on the sidewall of hole during screw insertion. Conventional drill screws do not form a complete hole during the drilling process since the screw begins insertion as soon as the threads engage a portion of the partial drilled hole.
Prior drill screws are designed such that their threaded section immediately begins to screw into the bore hole once the threads reach the surface of the work piece. It has been found that when the inner surface of the bore hole is rough, cracks are more likely to form in the work piece as the threaded section is screwed into the bore hole. Such roughening typically results from chips or other pieces of the work piece not being fully cut from the work piece and removed from the bore hole by the drill bit during the drilling operation (i.e., from the hole not being substantially reamed smooth).
Work pieces made of material such as fiberglass are particularly susceptible to such crack formation when the inner surface of their bore hole is not substantially smooth. When a bore hole is drilled through a walled work piece, it is often necessary to move the drill bit in and out of the bore hole numerous times or allow the bit to rotate in the hole numerous revolutions in order to obtain a bore hole with a such a smooth inner surface. Because prior drill screws are designed to drill and screw in the same direction of rotation, it is difficult to ream the bore hole smooth in this manner without at least partially screwing and unscrewing the threaded section of the drill screw in and out of the bore hole. Such screwing and unscrewing can result in a looser fit between the drill screw and the work piece and a corresponding drop in the integrity of the anchorage.
If chips or other pieces of the work piece material are not removed from the bore hole before the threaded section is screwed in, these pieces will be trapped within the hole and wedge between the threaded section and the work piece. Such a wedge tends to cock or misalign the drill screw relative to the bore hole and cause even greater stresses to be applied to the surrounding material. These greater stresses in turn increase the likelihood of cracks being generated and then propagated. In addition, especially when the work piece is made of metal material (e.g., steel), these trapped pieces can cause the threaded section to bind-up in the hole. This increases the likelihood of the drill screw being over torqued, ultimately causing the drill screw to fracture. This is especially true for aluminum or steel drill screws.
Therefore, there is a need for an improved drill screw capable of drilling a completed bore hole before anchoring the screw into the hole thus allowing fastening to a walled work piece without the use of additional tools further eliminating associated stress around the bore hole.